CETN-III-10
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along the shore to cause or maintain an accumulation alongside a groin or
between groins, or if it is filled artificially.
2. Groins that fill only by entrapment of littoral drift are likely to
increase the erosion rate along the shore for some distance downdrift of
the structure site, Unless the area downdrift can be sacrificed, filling
the groin system with imported sand at the time of construction, and in
some cases refilling it later, will be necessary.
3. A groin's length and height must be chosen to maximize the accumu-
lation on the updrift side, while minimizing the erosion downdrift. The
spacing of groins in a system must be selected to maximize sand accumu-
lation within the compartments.
The economics of reducing the number of
groins and their length must also be considered.
4. A groin must be stable against earth pressures created by the dif-
ference in sand levels on the two sides.
It must also be stable against
w a v e , current, and ice forces. Resistance can be developed by providing
rubble-mounds and cells with adequate weight and base width, or by pro-
viding sheet-pile structures with sufficient penetration.
The foundatlon
must be adequate to support the weight of gravity structures without
undergoing
unacceptable
settlement.
5. A groin must resist the scour created by waves breaking on the struc-
This scour can reduce the effective
ture and currents adjacent to it.
pile penetration, or can undermine rubble-mound structures.
To design
for scour, longer sheet-piles can be driven and toe-protection aprons can
be added for either sheet-pile or rubble-mound groins.
6. The groin must be constructed to prevent failure due to flanking.
The landward end must be extended into the shore or upland, or tied to
protective structures running along the shoreline.
MATERIAL SELECTION FOR GROINS:
1. Q u a r r y s t o n e , of suitable size and structural properties for use in
rubble-mound groins and scour aprons, may not be economically available
in all coastal areas of the United States.
Where available, quarrystone
may be the preferable material for groins, and especially if they are
Rubble structures
to be located in environmentally sensitive areas.
provide good habitat for marine organisms, while sheet-pile groins provide
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